Supported metal catalysts are used in a variety of processes in the chemical industry. Hydrogenation and dehydrogenation processes, cracking processes and oxidation processes are among such processes. The noble metals, especially silver, and the platinum group metals probably are employed most frequently in the above mentioned processes. Platinum has long been used in cracking, hydrogenation and dehydrogenation processes and silver is well known as a catalyst in the production of ethylene oxide by the direct oxidation of ethylene.
Various methods of depositing these metals on supports are known. Generally, the support is impregnated or coated with a solution of the salt of the metal or metals to be employed. This is followed by drying and a subsequent reduction. Although the reducing step can be a thermal reduction in the presence of air or an inert gas (U.S. Pat. No. 2,709,123), hydrogen and hydrazine (U.S. Pat. No. 3,575,888) have also been employed for the reduction. Reducing agents in the form of organic compounds have been added to the catalyst, either by simultaneous application to the support or subsequent addition to the impregnated metal salts. Also reducing compounds have been known to be employed by incorporating them into the support prior to the addition of the catalytic component.
Reducing agents frequently employed with silver salts include carboxylic acids or their salts and nitrogen-containing organic compounds, or ammonia, or a combination of the two as in U.S. Pat. No. 3,702,259. Other nitrogen-containing compounds, e.g. polyacrylonitrile (U.S. Pat. No. 3,892,679), an alkanolamine or an acid amide (U.S. Pat. No. 4,248,740) have also been employed. While the solvent used to apply the reducing agent is frequently water as in U.S. Pat. No. 3,702,259 above, organic solvents also have been employed, as for example, ethylene carbonate, dimethyl formamide and dimethyl sulfoxide as solvents for the polyacrylonitrile in U.S. Pat. No. 3,892,679, above. In the same patent the use of non-solvents to precipitate the polymer from solution onto the support is indicated and is illustrated by the use of methanol and toluene.